Cathode ray tube



June 20, 1939. A. B. DU MONT CATHODE RAY TUBE 2 Sheets-Sheet 1 Filed Sept. 18, 1954 REE zskomiwo wwom INVENTOR.

VOLTS ACROSS DEFLECTION PLATES S V E N1 R O A June 20, 1939. DU MONT 2,163,256

v CATHODE RAY TUBE Filed Sept. 18, 1934 2 Sheets-Sheet 2 INVENTOR.

O J Wad? ATTOhNEl'S Patented June 20, 1939 ,itazst CATHODE RAY TUBE Allen B. Du Mont, Upper Montclair, N. .L, assignor to Allen B. Du Mont Laboratories, llnc., Upper Montclalr, N. J a corporation of Delaware Application September 18, 1934, Serial No. 744,498

2 Claims.

This invention relates to cathode ray tubes of the type which are used for the analysis of wave form, for television reception or for measuring electrical values by causing a beam of electrons to trace a pattern on a fluorescent screen. In such tubes as heretofore constructed there have been certain distortions present due to various factors and the tubes have been subject to breakdowns from various causes which are remedied by the present invention.

One of the distortion effects which may be noted in the present cathode ray tubes and which is eliminated by the tube shown in this invention is what is generally termed the threshold effect. That is, in gas focus tubes with deflection plates, a certain amount of ionization is present clue to the collision of electrons in the projected beam with gas molecules in the space between the deflection plates. The effect of this is that a space charge of positive ions with relatively low mobility is present between the deflection plates and these ions effectively shield the beam from the influence of small negative potentials on one defleeting plate before linear response is established.

Another distortion effect is due to the variable impedance across the deflection plates as the deflecting potential varies. When a' positive voltage with respect to the accelerating electrode is applied to a deflection plate a much larger current flows to it than when a negative voltage is applied. For instance with a deflecting potential of plus one hundred and thirty-five (135) volts there might be a current of plus sixty-five (65) microamperes as against a current of minus two (2) microamperes with minus one hundred and thirty-five (135) volts. This effect is particularly objectionable when the tube is being used in conjunction with high impedance circuits as on the positive cycle it shunts the signal,- giving an incorrect indication. It is impossible to avoid such distortions in tubes as at present constructed but this effect is eliminated by the construction and operation of the tube in this invention and the impedance across the deflection plates is constant and high regardless of the deflection voltage applied.

A feature of the invention is a new geometrical arrangement of the structure of cathode ray tubes so that the threshold effect does not occur on the fluorescent screen and the deflection plates always operate at a negative voltage with respect has been deflected by the lower pair of plates, than by one which has not been deflected. The provision of specially shaped top deflecting plates which will have a uniform effect on the beam independent of the amount the beam has been deflected by the lower pair of deflection plates is a feature of this invention.

In previous types of cathode ray tubes a nickel filament has been used to which a short piece of nickel wire has been welded at a central point, and the tip of this short piece of wire coated with electron-emitting material to provide the source of electrons. With such a construction the normal current to the filament may be from 1.0 to 1.4 amperes and when this is exceeded the filament begins to sag. This causes the beam to get out of alignment or the filament to burn out.

An object of this invention is to provide a type of filament construction and support which will prevent sagging with much higher currents than those normally used, will provide a better spot, will eliminate burnouts, and will heat instantaneously.

Another feature of this invention is an insulating support for the filament legs and a brace from this support to the concentrating cylinder to retain the filament securely in position.

It is desirable for many purposes to be able to compare a number of waves simultaneously, and particularly to be able to compare the voltage and current of a single wave.

Another feature of this invention is the provision of means for producing a plurality of cathode beams in a single tube, with a single pair of deflection plates providing the sweep circuit, or time axis. Two cathode beams may be used and these may be focussed at a single point for comparing the phase relationship of the current and voltage of a single wave.

In the drawings,

Figure 1 is a diagram showing the normal axes of the tube of the prior art and the axes of the tube of the present construction;

Fig. 2 is a graph showing the relationship of current to voltage across the deflection plates;

Fig. 3 is an elevation of a tube showing the new geometrical arrangement and the new construction of deflection plates of the present invention;

Fig. 4 is a detail showing the top deflection plates of the tube of Fig. 3;

Fig. 5 is a view of a portion of a tube showing a plurality of cathode beams focussed to a single point with a single sweep element, the deflection plates of which are arranged in accordance with this invention;

Fig. 6 is a cross-section of the concentrating cylinder, showing the new filament structure of the present invention;

Fig. 7 shows a circuit for connecting the deflection plates so that a high constant impedance is obtained across the deflection plates; and

Fig. 8 shows -another circuit for accomplishing the same result.

In cathode ray tubes constructed in accordance with the prior art, the structure which projects the beam is mounted vertically on the press at the base of the tube. That is, it is mounted parallel to the axis of the tube. with such con struction the beam is of course focussed upon a point which is at the center of the screen, or the point at which the X and Y axes cross one another in Figure 1. In order to avoid the threshold efiect discussed above, and the variable impedance eflect also discussed above, and illustrated in Figure 2, I mount the beam projecting apparatus of my tubes according to this invention at an angle to the axis of the tube, The entire construction is simply swung over at the top at an angle of degrees to the X, Y axes so that the beam projected without any potentials applied to the deflecting plates would be focussed at a point where the X, Y axes of Figure 1, would join. Thus it will be seen that by applying an initial negative bias to the proper deflection plates of each pair as shown in Figures 8 and 9 the beam may be focussed again at the center of the screen and variations of voltage on the deflection plates, will produce the usual pattern at the same portion of the screen but without the distortion eflects referred to above as the threshold eflect will be moved off the fluorescent screen and the tube will operate on the portion of the curve A, B as shown in Fig. 2 instead of A, B as in present tubes. It will be noted that by swinging the beam projecting apparatus to an angle of 45 degrees, I employ only one quadrant of the beam range and that the screen is located within this quadrant.

In Figure '3 there is shown an evacuated or 'gasfllled tube I, mounted on a base 2, having apressi on which is mounted the beam projecting apparatus 4. In this beam projecting apparatus there is shown in the figure a iocussing.

or concentrating cylinder 5, ananode or accelerating electrode 8, a lower pair of deflecting plate I, and one of an upper pair of deflecting plates 8.

It will be noted that instead of the usual rectangular form of deflecting plates 8, I employ top deflecting plates having curved upper and lower edges. In Figure 4 I have shown the lower deflection plates 1 and one of the upper deflection plates 8 of Figure 3.

From Figure 4 it will be seen that if the deflection plates 8 were rectangular, the beam III which is not deflected by the plates 1, would traverse a shorter path through the plates 8 than would be traversed by the beams H and I! which have been deflected by the deflecting plates 1. The length of the path traveled by the beam through the deflecting plates 8 will, of course, be proportional to the amount oi deflection due to the deflecting plates 1. As the deflection is proportional to the length of the deflection plates it is apparent that with square or rectangular top plates the deflection at the center of the plates is less than at the edges. By shaping the plates so that the curvature is an arc of a circle about the point of initial deflection a true pattern results. In this manner a beam will traverse a path of equal length through the deflecting plates 8 regardless of the amount it has been deflected by the deflecting plates 1.

As previously mentioned it is desirable for some purposes to be able to compare directly two or more waves, or the voltage and current in a single Wave. In order to accomplish this I provide a tube having a plurality of electron beams subject to deflection by separate pairs of lower deflecting plates but which traverse a single pair of upper deflecting plates to provide a time axis. In Figure 5 I have illustrated a tube having two electron beams focussed at a single point by tilting the focussing assembly and two lower pairs of deflector plates, and a single pair of upper deflecting plates incorporating the curved feature previously described. Similar parts in all the figures are designated by similar reference numerals.

In Figure 6 is illustrated the Special filament construction whereby the improved results referred to previously are obtained. With this construction with a tube which has a normal rating of from 1.0 to 1.4 amperes, for example, I may employ as high as 3.0 amperes of current with less danger from burnouts than occur with 1.5 amperes of current with filaments oi the prior art. The filament l8, which may be of tungsten, is enclosed within an insulating support ll. This insulating support may be of the usual type of insulator which is employed in vacuum tubes having indirectly heated cathodes. That is, it is a small cylinder through which the filament wires pass. In vacuum tubes having, indirectly heated cathodes it is customary to place a metal sleeve around the insulator and coat this with electron emitting material. Ido not employ any such sleeve on my insulators. In fact, I do not use any form of coating on them, simply using them as insulators. Welded to the central point of the filament I3 is a short piece oi wire II which may be of molybdenum. The tip of this wire is covered with an electron emitting coating It to provide the electrons which constitute the beam. The insulator I4 is braced and secured within the concentrating cylinder 5 by means of a metal spacer H, or in some other suitable manner.

In Figs. 7 and 8 I have shown circuit arrangements for use with the tubes disclosed herein. In Figure 7 one of the lower pair of deflection plates is connected to the negative side of a volt battery, the positive side of which is connected' through a resistance to the other of, the lower plates. The midpoint of the resistance is grounded. Across this resistance a deflection voltage E is applied. One of the upper pair of deflection plates 8 is similarly connected to the negative terminal of a 135 volt battery, the positive terminal of which is connected through another resistance to the other of the upper deflection plates. The midpoint of this second resistance is also grounded and. acros the resistance a deflection voltage E is applied. The accelerating electrode 6 is also grounded. with this arrangement, if ammeters are introduced into the circuit as indicated by the letter A, it will be seen that there is one microampere of current flowing into the circuit of the lower pair of deflection plates, and two microamperes of current flowing in the circuit of the upper pair of deflection plates. The impedance of the circuits with this connection is high and constant, so that the tube operates with constant characteristics and at low current. If the voltages of the two batteries were reversed, so that the positive terminal of each battery were connected direct through the ammeter to the particular deflection plate, the current flow in the circuit of the bottom pair of deflection plates would be 10 microamperes and that through the upper pair of deflection plates would be microamperes. The reason for this difference in current with different connections of the batteries is that as the beam is projected to the screen there must be some return path, and this return path will naturally be to the point of highest potential. If the accelerating electrode is kept at a higher potential than the deflection plates the current will return to it instead of to the deflection plates. As one of each pair ofdeflection plates is at minus volts with respect to the accelerating electrode 6, and the other of each pair of deflection plates is also negative with respect to the accelerating electrode 6, the return path with connections shown in Fig. 7 is to the accelerating electrode 6, whereas if the voltages of the batteries are reversed the return path would be to the deflection plates. With the circuit shown in Fig. 7 signals'may be coupled to the tube through input circuits of equal impedance which are symmetrical about the earth potential.

In Fig. 8 is shown another circuit or arrangement in which the reference numerals and letters are the same as in Fig. 7. In this arrangement 3 microamperes of current will flow in the circuit. If the battery in this circuit were reversed there would be '75 microamperes of current in the circuit for the same reason as that explained in connection with Fig. 7. In the circuit arrangement of Fig. 8 it will be noted that only a single battery is used.

What is claimed is:

1. A cathode ray tube having beam producing means comprising a concentrating cylinder, an insulating cylinder having two holes therethrough, a spacer connecting said insulating cylinder with said concentrating cylinder, a tungsten wire filament threaded through the holes in said insulating cylinder, a molybdenum wire secured to said tungsten filament, and an oxide coating on the tip of said molybdenum wire.

2. A cathode ray tube having a fluorescent screen, means for producing a plurality of beams comprising a plurality of concentrating cylinders, a plurality of insulating cylinders, a plurality of spacers connecting said insulating cylinders with said concentrating cylinders, a plurality of fllaments threaded through said insulating cylinders, a plurality of wires secured to said filaments, said wires having electron emitting coating at the tips, an anode having a plurality of apertures therein, a plurality of pairs of lower deflection plates arranged to deflect the beams in one direction, a pair of upper deflection plates having curved edges arranged to deflect the beams in a different direction, said beam producing apparatus, anode and deflecting plate being arranged to focus said beams at a point off said screen and means for biasing said deflection plates to focus said beams on said screen.

ALLEN B. DU MONT. 

